The invention relates to a method and apparatus for the correction of image distortion in the digital transmission of video signals.
In the digital transmission of video signals in the digital telecommunications networks of the future, a bit error rate of about 1.times.10.sup.-6 is to be expected. Even at this low bit error rate, the brightness or the color of individual image points will be so distorted that the image quality will be noticeably impaired. On some sections of the transmission line the bit error rate may be substantially higher, for example 1.times.10.sup.-2. For the correction of the bit error itself, the error-correcting processes known from the field of data communication can be used. In this case it is sufficient to provide protection for the bits with the highest value of the image signal values to be transmitted. A disturbance of the bits with low values produces only a slight distortion of the color or brightness and therefore appears to be less troublesome. A disadvantage in the use of a conventional distortion correction procedure is the relatively high redundance, which results in an increase in the transmission rate. In addition, the correction capacity is exceeded even at relatively low bit rates, which may actually result in making the image quality worse than in a system without distortion correction.
Depending on the coding selected, substantial switching procedures are often necessary.
One correction circuit is known from U.S. Pat. No. 4,376,289; however, there is no means of identifying or predicting sharp contrasts in brightness which are truly present.